The invention relates to heat shielding or conserving means and to panels for such means, and it is concerned particularly, but not necessarily exclusively, with heat shielding or conserving means for use in metal processing.
In steel mill processing whether of billets, strip or sections, the metallurgical qualities of finished product are closely related to the accurate control of temperature of the material during the hot rolling process. For example, a modern hot strip mill producing steel coil is several hundred metres long and typically, steel slabs or billets may be reduced from 25 cms thickness to 0.2 cms using several roughing mill stands and five or more finishing mill stands. During the rolling, process considerable heat losses occur so that the slabs have to be heated initially well in excess of the temperature requirement at the end of the process, but a particular problem has been that the heat losses from slabs passing along the mill depend upon the time taken. If the slabs are delayed, excessive heat losses occur and the steel strip does not have its required rolling temperatures, so that it may have to be downgraded or even scrapped. In many long modern rolling mills the delay of one length of strip at the finishing end has an effect on the several lengths of material which are simultaneously at various preceding stages of rolling. Thus with more stringent quality specifications it is becoming more important to reduce the rate of heat loss from the material during transport between stands.
There is an added difficulty in this because during the final reduction stages the back end of the strip takes longer to pass through the finishing mills and so there is a temperature "run-down" along the steel strip due to the cumulative time delay along the length of the strip. The effect of temperature "run-down" is to some extent ameliorated by accelerating the finishing mills during the rolling of each individual slab or strip, but nevertheless it remains a problem.
Attempts have been made in the past to reduce the heat loss from the top surface of a hot strip during transport from the roughing mills to the finishing mills. Because radiation is a major source of heat loss at the temperatures involved (around 1060.degree. C.) aluminium reflectors have been fixed over the path of the hot strip to reduce temperature "run-down". However maintenance problems limit the usefulness of reflectors which become inefficient as soon as they become dirty, and in addition, the aluminium reflectors which have been used for their high reflectivity and relatively low cost can reach their melting temperature if their reflectivity decreases.
It has been proposed (UK Pat. No. 1 040 420) to use, as heat shields above the hot strip in a hot-rolling mill, heat reflecting panels made up of two similarly corrugated thin plates, the plates being arranged one over the other with the corrugations at right angles. Thermal insulation material is put on the upper face of each plate to reduce heat losses. This proposal offers a theoretical advantage in that it provides a re-radiating surface that heats up quickly to a luminous temperature close to that of the hot strip and so inhibits radiation from the strip, but the use of such panels raises practical problems.
To give the required structural stiffness to two thin plates with transverse series of corrugations, the plates must be secured together at the points of contact of their mutually transverse corrugations. Unless they are attached thus they cannot provide a stable planar structure. In principle, the individual corrugations are then still able to flex to accommodate relative expansion effects when the panel is heated, but under the very severe conditions encountered in steel rolling mills, particularly because of the need to have very rapid heating of the panels over a very large temperature range if they are to perform their function, the mechanical strains and the high maximum temperatures impose stresses on the point attachments that very quickly lead to fracture and to separation of the plates. The use of thermal insulation over each corrugated plate as proposed by UK Pat. No. 1 040 420 makes the problem worse as this increases the temperature differentials between the upper and lower plates and therefore adds to the mechanical strain upon the point attachments.
The very large and rapid temperature variations in operation and the resulting considerable thermal expansion movements can also quickly affect the thermal insulation material, having regard to the fact that its expansion co-efficient will be very different from that of the metal plates. This can result in displacement of the material that gives a non-uniform insulating effect adversely affecting the efficiency of operation, and it can also considerably reduce the operational life of a panel as the substantial relative thermal movements are likely to cause mechanical damage to the insulation.
Another source of non-uniform heating effects arises from the fact that, in order to reduce costs, the individual panels of an installation should be made as large as possible, but the extended use of such panels in severe conditions, in particular when large thermal gradients are experienced, can result in difficulties. These gradients may occur as a hot billet or slab is introduced, or because only part of the transverse extent of the panels is covered when rolling narrower strip. For example, in large panels (e.g. exceeding 400 mm by 400 mmin plan area) expansion of the plate forming the hot face of the panel can be so great that gross buckling can occur, and the plate may be cracked by the repeated stresses as the panel is subjected to successive thermal cycles from about 300.degree. C. to 1050.degree. with the movement of each hot slab past it.
If panels are also provided to shield the underside of the material path through the rolling mill, these may be subject to an additional difficulty due to the build up of foreign matter on them. When steel products are reheated, e.g. to temperatures up to 1250.degree. C., scaling of the slabs or billets will occur (i.e. oxidation of the surface to form loosely attaching scale). For many products, scaling is acceptable and is even desirable because it removes minor surface defects and results in a better finished product, but there is a problem in disposing of the scale if heat shielding panels are arranged below the material path. Typically, furnace oxidation can result in the formation of scale to the extent of some 11/2% of the product itself and most of this scale will fall from the product during rolling.
The deposition of scale at this rate on panels below the material can quickly result in a build-up or deposit which effectively acts as a thicker wall than the casing plate forming the hot face of the panel. This effect is undesirable because it alters the thermal characteristics and physical properties of the panels, which will have been designed to suit each particular installation. It is desirable therefore to avoid any significant accumulation of scale on the panel. A measure of the problem is that in a mill rolling about 400 tonnes of steel per hour, up to 2 tonnes of scale per hour can be deposited over a 60 meter long installation.